1. Field of the Invention
The present invention relates generally to a digital temperature sensor. More particularly, the present invention relates to a digital temperature sensor digitally displaying a temperature measured depending on a slight variation in a resistance value without using an analog-to-digital converter, and a system and a method for measuring a temperature using the digital temperature sensor.
2. Description of the Related Art
Temperature sensors are widely used in various industry fields. For example, temperature sensors are used to precisely produce products in precision industries, such as the semiconductor industry, as well as to control a temperature in electric home appliances such as air conditioners, refrigerators, or the like.
Currently used thermometers are classified according to the following measurement principles.
There are thermometers which use thermal expansion, for example, gas thermometers, liquid thermometers, and bimetal thermometers. The liquid thermometers include thermometers using mercury or kerosene. In the past, most alcohol thermometers (using alcohol dyed red) have red liquid columns. However, condensation easily occurs in an upper space of the liquid column, which generates an error in an indicator. Thus, the liquid thermometers have used kerosene for the past 30 years. Liquid thermometers include home thermometers clinical thermometers, maximum-minimum thermometers and Beckman's thermometers. The bimetal thermometers use bimetal made by adhering two types of metal plates having different expansion coefficients, i.e., copper and nickel thin plates. In the bimetal thermometers, the expansion coefficient of the copper is greater than that of the nickel. Thus, when a temperature is high, the nickel plate is bent. When the temperature is low, the copper plate is bent. The bimetal thermometers display the temperature using this principle.
There are thermal resistance thermometers using temperature variations in an electric resistance. The thermal resistance thermometers include resistance thermometers using a property that electric resistance values of metal and semiconductor depend on a temperature.
In thermocouple thermometers, both ends of two types of metals or alloy wires contact to make a loop-shaped circuit through which electricity flows. If a temperature difference is given to the contacting ends, a thermoelectromotive force is generated at the contacting ends, and thus a current flows in the contacting ends. This is called Peltier effect (thermoelectric effect). The thermocouple thermometers utilize the Peltier effect. Potentiometers or millivoltmeters having great internal resistances measure the thermoelectromotive force and are widely used for measuring a temperature during compensation of the temperature and the thermoelectromotive force. This is because of a small error in heat capacity and a superior response to heat due to the very small volume of a metal junction that senses humidity. Examples of a thermocouple include a couple of platinum wire and alloy wire of platinum and rhodium, a couple of copper wire and constantan wire, and the like.
There are color thermometers depending on light. Examples of the color thermometers include optical pyrometers and radiation pyrometers. The optical pyrometers compare color temperatures of objects to be measured with a standard color temperature to measure a temperature, i.e., may measure a color temperature between 700° C. and 2500° C. In the radiation pyrometers, thermal energy radiated from an object to be measured is condensed using a lens or a concave mirror and a thermistor (a resistor sensitive to a room temperature) is put on a focus to measure a temperature depending on variations of a resistance value caused by a rise in the temperature of thermistor. An example of the radiation pyrometers includes thermography thermometers using a semiconductor thermosensitive device for infrared rays. A distribution of a surface temperature of the earth or the skin temperature of the human body is investigated from an artificial satellite using the thermography thermometer.
There are segercone thermometers and thermocolor thermometers. The segercone thermometers are about-10-cm-triangle cones made by kneading silicate and metal oxide. The segercone thermometers are used to investigate temperature distribution in a furnace by disposing and heating triangle cones and observing a melting degree of the triangle cones. The thermocolor thermometers use a principle of a change in a color of a thermocolor called heat sensitive paint. The thermocolor thermometers use a phenomenon of a reversible change in a color of complex salt such as cobalt, chrome, or the like depending on temperature. The complex salt is kneaded with clay and then dried to make thermoclay. Liquid crystal thermometers using a temperature characteristic of a liquid crystal are recently released as the thermocolor thermometers.
The thermal resistance thermometers use a property that an electric resistance of a conductor varies with variations in temperature, i.e., a property that a corresponding temperature can be measured using a variation rate of a resistance with respect to variations in unit temperature. The variation rate of resistance with respect to the unit temperature is called a temperature coefficient of resistance (TCR). If a resistance value is increased with an increase in temperature, the variation rate of resistance is called a positive TCR. If the resistance value is reduced, the variation rate of resistance is called a negative TCR. A metallic material used for measuring a temperature mainly has a positive TCR. Examples of the metallic material include platinum, nickel, copper, and the like. As a material is pure, a TCR of the material is increased and constant. William Siemens first manufactured a resistance thermometer using platinum to measure a temperature in 1871. As previously described, resistance values of all kinds of metals depend on temperature as shown in Table 1 below.
TABLE 1MaterialResistance (uΩm)TCR (ppm/° C.)Carbon (graphite)1,390−500Manganin (alloy)48.22Nichrome1011,700Chromium12.93,000Aluminum2.833,600Silver1.633,800Copper1.723,900Platinum10.63,927Tungsten4.204,500Iron9.716,510Nickel6.846,900Gold2.408,300
Sensors made of a metal such as nickel, copper, platinum, or the like and an alloy have been developed using a dependence of a resistance value of a metal on temperature as shown in Table 1. However, platinum is mainly used due to its superior stability and good TCR. A platinum temperature sensor is used as a standard temperature sensor in a temperature area between −260° C. and 630° C. due to its high accuracy.
FIG. 1 is a block diagram of a conventional digital temperature measuring system using a platinum resistance. The conventional digital temperature measuring system includes stationary and variable resistors R0 and R(T) connected to each other in serial, a direct current (DC) offset removing circuit 10, an amplifier 20, an analog-to-digital converter (ADC) 30, a digital display 40, and a controller 50.
The stationary resistor is set so that its resistance value R0 is kept at a reference temperature T0, mainly at a room temperature of 20° C., and the variable resistor is set so that its resistance value R(T) varies with a temperature. The resistance value R(T) of the variable resistor at a temperature T° C. can be expressed as in Equation 1:R(T)=R0[1+α(T−T0)]  [Equation 1]where R(T) denotes a loadless resistance value Ω of the variable resistor at the temperature T° C., R0 denotes a loadless resistance value Ω of the variable resistor at the reference temperature T0, α denotes a TCR ppm/° C. or ppm/K, T denotes a substantially measured temperature ° C. or K of the variable resistor, and T0 denotes the reference temperature ° C. or K.
For example, if a platinum resistor having a TCR α of 3900 ppm/° C. and a resistance value of 1 kΩ at a room temperature is used as a temperature sensor, it is supposed that a voltage of 1V is applied to both ends of the platinum resistor at a power voltage VDD of 2V and the temperature is increased by 1° C. According to Equation 1, R(21° C.)=1 kΩ[1+0.0039]=1.0039 kΩ. In other words, as shown in FIG. 2, the platinum temperature sensor has a resistance of 1 kΩ at a temperature of 20° C. However, a resistance component of 39Ω is added to the resistance value of 1 kΩ at a temperature of 21° C. Thus, the controller 50 controls the digital display 40 to display a temperature of 20° C. when the voltage of 1 V is output but to display a temperature of 21° C. when a voltage of about 1.0019V is output due to a change of the resistance value of the platinum temperature sensor to 1.0039 kΩ.
Such a digital temperature measuring system displays a temperature based on variations in an output voltage caused by variations in a resistance value of a platinum resistor. However, in the digital temperature measuring system, a signal variation in an output voltage caused by variations in the resistance value of the temperature sensor is slight. Thus, the probability that an error occurs is very high. Also, the digital temperature measuring system requires a low noise circuit and a high-performance amplifier for removing DC offset prior to ADC conversion.